I get really dizzy & my vision goes white (can't see), usually when i stand up. sometimes my knees buckle, i collapse. usually only for 10~20 secs. y?

Orthostasis: This is a phenomenon called orthostasis. It is very common and usually related to blood volume status. The reason you are having this is the unclear part and what a doctor can help you with. Try doing 5 toe raises when you feel the symptoms to raise your blood pressure

Postural hypotension: Feeling lightheaded on standing , having blurry vision and collapsing is most like drop in blood pressure , called postural hypotension. In most cases it does not indicate anything sinister but falls secondary to it can cause injury . Certain blood pressure medications, dehydration due to diarrhoea, vomiting can also result in postural hypotension. See Gp for review if symptoms occur regularly.